TDI hot start issues

[Mechcanico Lee]

Hello , rail pressure will fade pretty quickly, this is called pressure decay and is normal , yes there very often is a check valve or non return valve in the system but this is just to keep the low pressure primed up to the high pressure pump to stop ' drain back ' .
Actual fuel pressure will always follow the desired pressure or be pretty close throughout the load range .

300 bar at idle sounds about correct , 10 % duty cycle sounds about correct for the idle aswell , the inlet metering valve must be the ' fail to closed type ' so in its un powered state the valve is closed ....but it's not fully closed some fuel can bypass it and this would normally give a default pressure of 80 bar .

Now ,when the car does the non start instance what is the pressure ? .....around 80 bar ...... coincidence or is there something in this ??

I can see that the imv duty cycle is going higher trying to build the start up pressure of around 300 bar on the graph but after 11 seconds of cranking time the rail pressure has not risen .

So what could cause this ..... rail pressure loss due to spilling injectors , intank pump not running on cranking or not making pre pump pressure. Imv fault or valve not being driven for whatever reason .

Normally with excessive back spill the car struggles a lot from cold which originally we did not have , without a guage in low pressure line we cannot fully rule out low pressure feed .

At this point I have a duty cycle tool I can plug into the imv and I can take over manual control of the imv and drive it to where I want to and see what happens with the pressure ....you have to put an emulator in the loom plug for the imv so the ecm still thinks it's connected to the car .

Looking at rail pressure whilst in non start condition it does not even fluctuate, it's as though the valve is stuck .

If you could get someone else to do the cranking when it's in a non start condition and give that imv a clonk with a spanner see if anything happens .

Just try this test .... have rail pressure showing on vcds un plug imv and crank over and see what rail pressure shows .

It could be a faulty imv but without some of the proving test gear in place the diagnostic pathway tests are not conclusive , so it is more a calculated guess with this one .

Keep at it , your getting closer to a fix